More than 3,000 infants in Brazil have been diagnosed with brain deformities believed to have been triggered by the Zika virus, a disease similar to dengue fever that has become increasingly common as the Southern Hemisphere’s summer wears on.

Brazil’s Ministry of Health has recorded 3,530 cases of microcephaly in infants that it believes is the product of the mother’s contraction of Zika virus, Brazilian newspaper O Globo reports. Children with microcephaly are born with smaller brains and skulls, leading to a variety of mental disorders and, in some cases, death. Brazilian officials warned that they had recorded 356 suspected cases of microcephaly linked to Zika in the past week alone.

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The Wall Street Journal notes that Brazilian officials appear significantly confident that Zika is to blame for these cases of microcephaly. “Before the explosion of cases since mid-2015, Brazil had an average 150 cases of microcephaly a year. Today I can tell you that we have 100% certainty of the connection of the Zika virus with increasing cases of microcephaly in Brazil,” Brazilian Health Minister Marcelo Castro told the newspaper.

Brazilian scientists announced this week that they have confirmed at least four cases in which the mother’s contracting of Zika virus led to her child being born with microcephaly. In addition, researchers are finding preliminary ties between Zika and another birth deformity: arthrogryposis, a disorder which creates joints incapable of full mobility, shielded by weak muscles. Doctors have linked at least two cases of arthrogryposis with Zika, both in pregnant women who have yet to give birth.

The Zika virus is transmitted through bites from the mosquito species aedes aegypti, which is also responsible for carrying diseases like dengue fever, chikungunya, and yellow fever. The species lives only in southern regions and thrives in humid climates. Given the presence of the El Niño phenomenon this year, Brazilian authorities have been expecting a spike in Zika cases, with O Globo going so far as calling this season the “summer of Zika.”

Among adults, Zika is rarely deadly and can only be treated through giving a patient fluids and waiting for the virus to subside. Its impact on in-utero children has triggered an international alarm, however.

At least two cases of Zika have been documented in the United States—in Texas and Puerto Rico—this year, though neither appear deadly and children are not involved.

In December, Brazilian health authorities began suggesting to couples planning on having children to delay their attempts to conceive as long as possible, or at least until the Zika threat subsides. “It’s a very personal decision, but at this moment of uncertainty, if families can put off their pregnancy plans, that’s what we’re recommending,” Angela Rocha, pediatric infectologist at Oswaldo Cruz Hospital in Pernambuco, told CNN.